Finned and pinned heat sinks are commonly used for enhancing heat transfer in power electronics and microelectronics applications. The use of finned/pinned heat sinks reduces the thermal resistance and operating temperatures of components and assemblies by increasing the available surface area for convective heat transfer.
Typically the heat sink is a parallel or staggered array of thin flat plates or pins attached to a single, thermally conductive baseplate. The overall performance of a pin-fin heat sink depends on a number of parameters including the dimensions of the base-plate and pin-fins, pin-fin density, longitudinal and transverse spacings, the thermal conductivity of the material, approach velocities, and the geometric arrangement of pins or fins. Heat transfer from the sink occurs via free or forced convection in the channels formed between adjacent fins or pins. When forced, air flow is generally in a direction parallel to the base plate. The heat transfer rate from the heat sink is largely determined by the amount of flow through the heat sink, which is determined by the ratio of the heat resistance to the flow resistance of the surroundings. Higher flow resistances such as those which occur in staggered arrays will have less flow entering the heat sink, and may experience higher degrees of bypassing flow. Such bypassing flow is particularly relevant to the performance of pin arrays, which are often utilized in applications where the direction of the approaching flow is unknown or may change. The higher flow resistances also generally result in a higher proportion of the flow escaping through the top and sides before reaching the outflow side. These issues are mitigated when the finned or pinned heat sink is shrouded and the flow is confined, however high resistances still depress flow and thermal performance suffers.
The balance between flow and geometry in a heat sink is difficult to optimize, because increasing fin density increases the surface area for heat transfer, but also complicates the flow resistance and may depress flow to the point where overall heat transfer decreases for a given pumping power. Pumping power is generally parasitic and for forced convection applications, minimizing flow requirements based on a maximum allowable temperature in a heat sink is typically the goal. As a general statement, increasing the number of rows of pins or fins that a flow is expected to traverse can be expected to increase overall heat transfer coefficients while also increasing pressure loss through the heat sink and increasing the pumping requirement. Further, boundary layer growth among fins impacts nearest and downstream neighbors and may adversely affect the heat transfer coefficient of a given fin in the array. Typically in a multi-row fin or pin array, the highest heat transfer coefficients are found in the first row of fins. Some degree of impingement flow is typically occurring on the first row fins, breaking down boundary layer growth.
As is understood, flow through the heat sink and the resulting heat transfer coefficients is only one aspect of the overall thermal network governing heat sink performance. The thermal network of a finned heat sink is made up of conductive, radiative, and convective resistances as, from the junction of the device, heat is transported by conduction from the device through the interface and into the heat sink, where heat is usually removed mainly by means of convection via the air flow, as well as some typically negligible radiation cooling. The conductive resistance is a frequent area of concern and significant effort is aimed at reducing the material resistances of base-plates and pin-fins, contact resistances between a base-plate and a pin or fin, and fin-pin resistances. Material resistances are typically a function of geometry and material properties and are generally manipulated by those factors. Contact resistances may be impacted by many additional factors, such as surface irregularities at the interface, the geometry of the interfacing joint, adhesives utilized at the joint, and other influences. One effective method of eliminating the contact resistance is machining the pin-fins as an integral part of the base, however this can present fabrication difficulty, particularly as the size of heat sources and the heat sinks required to service them continue to decrease. Fin-pin resistances are similarly influenced by many factors, however fin-pin resistances are generally an inverse function of the fin-pin base area, so that maximizing the fin-pin base area in light of other prevailing heat sink factors is typically desired.
A variety of fabrication methodologies are employed in the production of pinned or finned heat sinks. Often the fins or pins and the base-plate are manufactured as separate elements, then fitted at an interface such as a fin slot or attached via brazing or some other method. These methods may increase contact resistances as discussed supra, and may become increasingly difficult as heat sink dimensions decrease. Alternate methods such as injection molding, etching, and laser melting are used to fabricate the fins-pins and base-plate as integral units, however fabrication ease greatly suffers. Additionally, in state-of-the-art heat sink fabrication methodologies, fabricating fins-pins and base-plates as integral units can be exceedingly difficult when the fins or pins may be required to have more complex cross-sections to enhance convective heat removal, such as elliptical, helical, or similarly shaped cross sectional profiles.
It would be advantageous to provide a heat sink addressing the various short comings of current heat sink technologies. A heat sink whereby flow geometries are designed in such a manner that flow resistance for a given fin density is minimized would act to increase the overall heat transfer capabilities of the heat sink while minimizing any parasitic pumping power required. Further, a heat sink providing a flow geometry whereby first row fins are maximized would limit boundary layer growths and increase average heat transfer coefficients across the fins. A heat sink providing a flow geometry which allows a high fin base area in contact with the base-plate would provide further advantage by decreasing fin thermal resistances. Additionally, providing a heat sink geometry where the fins can be easily fabricated as integral units with the base-plate would reduce contact resistances and improve overall performance. It would be further advantageous if the heat sink could be manufactured as a series of interleaved sections, so that the fin and base plate could be fabricated integrally in each section, and the composite heat sink having fin integral with the base-plate could be easily assembled. It would be further advantageous if such as arrangement provided for the servicing of multiple heat sources from a single and centralized fluid channel, in order to reduce the physical footprint of the heat sink in an operating application.
Accordingly, it is an object of this disclosure to provide a fluid-cooled heat sink having a flow geometry which minimizes flow resistance for a given fin density in order to minimize pumping power requirements.
Further, it is an object of this disclosure to provide a fluid-cooled heat sink having a flow geometry whereby first row fins are maximized, in order to limit boundary layer growths and increase average heat transfer coefficients across the fins.
Further, it is an object of this disclosure to provide a fluid-cooled heat sink having a flow geometry allowing a high fin base area in contact with the base-plate, in order to decrease fin thermal resistances.
Further, it is an object of this disclosure to provide a fluid-cooled heat sink whereby the fins can be easily fabricated as integral units with the base-plate, in order to reduce contact resistances and improve overall performance.
Further, it is an object of this disclosure to provide a fluid-cooled heat sink which may be manufactured as a series of interleaved sections, so that the fin and base plate can be fabricated integrally in each section, and the composite heat sink having fins integral with the base-plate can be easily assembled.
Further, it is an object of this disclosure to provide a fluid-cooled heat sink whereby the flow geometry further allows for the servicing of multiple heat sources from a single and centralized fluid channel, in order to reduce the physical footprint of the heat sink in an operating application.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.